A typical inverter converts direct current from a direct current supply to alternating current by an inverter circuit with bridge-connected switching devices, to have alternating current outputs. For this, the load does not always have a power factor of 1, and a free wheeling diode is connected in reverse parallel to a respective switching device. In this configuration, as the switching device turns off, stored energy in the load flows back through the free wheeling diode.
For a free wheeling diode conducting a forward current, if a switching device turns on in an arm with an opposite polarity to the free wheeling diode, a direct current voltage is applied as a reverse bias across the free wheeling diode. The free wheeling diode cuts off after a reverse current caused by residual charges flows therethrough. Hence much loss is caused at the free wheeling diode by the reverse current and direct current voltage, with a worsened efficiency of the inverter. Further, for this reason, there has been a need for an enlarged scale of cooler for the system.
For that, there has been a method disclosed in Japanese Patent Application Laid-Open Publication No. 10-327,585, on pages 3 to 5 and in FIG. 1, in which an inverter is provided with a reverse voltage application circuit, and upon a cutoff of a free wheeling diode, a small reverse voltage is applied from the reverse voltage application circuit to the freewheeling diode, so that a reverse recovery of the free wheeling diode is induced by a low-voltage direct current supply of the reverse voltage application circuit, so as to reduce the loss being caused at the free wheeling diode.
According to the above-noted preceding technique, the reverse voltage application circuit has a reverse voltage applying switching device, and upon a reverse recovery of the free wheeling diode, the reverse voltage applying switching device is turned on for application of a voltage of low-voltage direct current, thus permitting the free wheeling diode to be reverse recovered with a reduced loss.
However, in the preceding technique, the reverse voltage application circuit has an auxiliary power supply composed of capacitors and diodes, and drives switching devices for reverse voltage application by using the auxiliary power supply, so the configuration of the reverse voltage application circuit is complicated, causing also the volume of the reverse voltage application circuit to be enlarged, as a significant issue. Further, for capacitors to be charged, currents are conducted through diodes, not simply causing heat to dissipate as losses by those currents at the diodes, but also having the capacitors charged with voltages reduced lower than voltages of low-voltage direct currents by fractions of the losses at the diodes, as an issue.